Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 9,673	$ 12,500	$ 25,241
Foreign currency translation adjustments	9	(48)	(141)
Ending Balance	9,409	9,673	12,500
Cumulative Effect, Period of Adoption, Adjustment [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(2)		29
Ending Balance		(2)
Foreign currency translation [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(718)	(670)	(529)
Foreign currency translation adjustments	9	(48)	(141)
Income taxes	0	0	0
Ending Balance	(709)	(718)	(670)
Cash flow hedges [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	175	241	(6)
Unrealized (losses) gains	(61)	127	61
Reclassification adjustments to income	501	211	262
Income taxes	124	18	(76)
Ending Balance	(263)	175	241
Available-for-sale securities [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	22	(338)	(144)
Unrealized (losses) gains	6	424	(556)
Reclassification adjustments to income	33	56	365
Income taxes	6	(8)	6
Ending Balance	1	22	(338)
Available-for-sale securities [Member] \| Cumulative Effect, Period of Adoption, Adjustment [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(9)
Other [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(7)	(2)	0
Other losses	(7)	(5)	(2)
Ending Balance	(14)	(7)	(2)
AOCI [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(528)	(769)	(679)
Foreign currency translation adjustments	9	(48)	(141)
Unrealized (losses) gains	(55)	551	(495)
Reclassification adjustments to income	(534)	(267)	627
Other losses	(7)	(5)	(2)
Income taxes	130	10	(70)
Ending Balance	$ (985)	$ (528)	(769)
AOCI [Member] \| Cumulative Effect, Period of Adoption, Adjustment [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			$ (9)

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Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature, attributable to parent entity.

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